The present invention generally relates to electrical filters and in particular to demultiplexers for frequency division multiplexed (FDM) signals.
Filters for electrical signals exist in both analog and digital form. Analog filters are constructed with components whose number and precision increases with the desired precision in the filtered signal. Digital filters can provide precise filtering but require a large number of samples, and thus a high sampling rate, when a large bandwidth of signals is to be filtered. Accordingly, there is a trade-off between digital and analog filters depending upon the application for which the filtering is to be used.
Frequency selective filters and down converters have been used for many years in communication applications. An example of a simple unit of this type is the standard AM broadcast receiver. In the communications world, a typical application is the demultiplexing of frequency division multiplexed (FDM) signals. These signals are created by single-sideband modulation (conversion) of 4 kilohertz (kHz) bandwidth channels and placing the modulated signals in a channelized spectrum relative to a carrier frequency. The function of the demultiplexer is to select one of a set of modulated signals that have been multiplexed together in frequency, convert that signal to its original baseband form, and output the baseband signal either in analog or digital form. In analog technology, a typical implementation is the use of a double superheterodyne single-sideband receiver. Such an implementation requires complex crystal filters. The incoming signal is mixed with a controlled oscillating signal to produce a higher frequency signal which will be within the range of the crystal filter for the demultiplexer. The particular frequency to be received can be selected by tuning the oscillator for the controlled frequency signal so that the resultant heterodyned signal of higher frequency will be within the range of the filter. The filtered signal is then converted down to the baseband audio frequency range where the signal can be understood by a person.
A separate analog filter is required for each channel. Cumulative bandwidths on the order of 15 megahertz (MHz) are typically obtainable.
When a digital filter is used instead, the signal must be converted from an analog signal to a digital signal and then passed through a digital filter. The digital filter can be constructed using fast Fourier transforms in which the signal is broken down into its separate frequency components and each component is multiplied by a predetermined coefficient to produce the filtering desired. The drawback of digital filtering is the large number of computations required, which limits the bandwidth attainable. Bandwidths for digital implementations are typically limited to approximately 1 megahertz with currently available hardware.